Glass-ceramics for a light filter

ABSTRACT

There is provided a glass-ceramic which is suitable for use as a light filter. The glass-ceramic has Young&#39;s modulus (GPa) within a range from 95 to 120 and includes 5.3-8 weight percent of Al 2 O 3 , 0.5-3.5 weight percent of ZrO 2  and 71-81 weight percent of SiO 2 , based respectively on the total content of the oxides. The glass-ceramic preferably has, as its predominant crystal phases, (a) lithium disilicate (Li 2 O. 2 SiO 2 ) and (b) at least one of α-quartz (α-SiO 2 ) and α-quartz solid solution (α-SiO 2  solid solution), has specific gravity within a range from 2.4 to 2.6 and has a coefficient of thermal expansion within a range from 65×10 −7 /° C. to 130×10 −7 /° C. within a temperature range from −50° C. to +70° C.

This application is a division of Ser. No. 09/594,105, filed Jun. 14,2000, now U.S. Pat. No. 6,582,826, which is a continuation-in-part ofSer. No. 09/260,768, filed Mar. 2, 1999, now U.S. Pat. No. 6,383,645,which claims priority from JP-10-094020, filed Mar. 23, 1998;JP-10-125316, filed Apr. 20, 1998 and JP-10-351682, filed Dec. 10, 1998.

BACKGROUND OF THE INVENTION

This application is a divisional application from U.S. Ser. No.09/594,105 which is a continuation-in-part application from U.S. Ser.No. 09/260,768 filed on Mar. 2, 1999.

This invention relates to novel glass-ceramics and, more particularly,to glass-ceramics suitable for use as a light filter and, moreparticularly, to glass-ceramics suitable for use as a band-pass filterand a gain flattening filter.

There are light filters which cut or pass light of a specific wavelengthand there are also light filters which reduce intensity of light withoutdepending upon wavelength. The former includes a band-pass filter whichpasses only a specific wavelength, a notch pass filter which cuts aspecific wavelength and high-pass and low-pass filters which pass onlywavelengths shorter or longer than a specific wavelength. The latterincludes an ND filter.

Light filters can be classified also into an absorption type filter andan interference type filter. A representative absorption type filter isthe ND filter and a representative interference type filter is theband-pass filter. A substrate made of plastic is used for absorptiontype filters such as those for photography. Since a substrate for lightfilters which are subject to a strong laser beam requires durability andheat resistance property, amorphous glass is exclusively employed forsuch substrate.

The band-pass filters are made by forming, on a substrate made of, e.g.,glass, a multi-layer film of dielectric by alternately laminating an Hlayer of a dielectric thin film having a high refractive index and an Llayer of a dielectric thin film having a low refractive index.

In a band-pass filter which is used for the WDM(wavelength divisionmultiplexing) optical communication system, temperature stability of thecenter wavelength of the band poses a problem when a narrow band widthfor passing wavelengths is set for applying the band-pass filter to awavelength of a higher density. More specifically, the band-pass filteris a sensitive element in which the center wavelength of the band varieseven with a slight variation in temperature and, therefore, temperaturecompensation should be made by a temperature controller when theband-pass filter is used. Such temperature controller, however, cannotactually be employed because of limitation in the space where theband-pass filter is located. The temperature stability has become amatter of increasing importance since it is necessary to reduce the bandwidth as the amount of light information increases.

In the past, amorphous glass has been used as a substrate for theband-pass filter. This prior art substrate is not sufficient in itscompressive stress to the film and its durability since its thermalexpansion property and mechanical strength are not sufficiently high.Further, amorphous glass has low mechanical strength and therefore tendsto produce micro-cracks in processing with resulting cracking orchipping off of corner portions of the product which reduces the yieldof the product. Moreover, in amorphous glass, a relatively large amountof alkali ingredient must be added if a high thermal expansion propertyis to be provided and this poses a problem of elution of alkaliingredient during and after forming of the dielectric film on thesubstrate. Thus, amorphous glass cannot sufficiently satisfy the demandsfor a substrate for a light filter, particularly a substrate for aband-pass filter.

Known in the art are some glass-ceramics. For example, theglass-ceramics of a SiO₂—Li₂O—MgO—P₂O₅ system disclosed in U.S. Pat. No.5,626,935 containing lithium disilicate (Li₂O.2SiO₂) and α-quartz(α-SiO₂) as main crystal phases is an excellent material as a materialtextured over the entire surface in which, by controlling the graindiameter of globular crystal grains of α-quartz, the conventionalmechanical texturing or chemical texturing can be omitted and thesurface roughness after polishing (Ra) can be controlled within a rangefrom 15 Å to 50 Å. In this glass-ceramic, however, no discussion orsuggestion is made about Young's modulus and a coefficient for thermalexpansion which are important features of the present invention.

Japanese Patent Application Laid-open Publication No. Hei 9-35234discloses a magnetic disk substrate made of a glass-ceramic of aSiO₂—Al₂O₃—Li₂O system having predominant crystal phases of lithiumdisilicate (Li₂O.2SiO₂) and β-spodumene (Li₂O.Al₂O₃.4SiO₂) which has anegative coefficient of thermal expansion. This glass-ceramic has acomposition which contains a relatively large amount of Al₂O₃ ingredientand in which growth of SiO₂ crystals such as α-quartz (α-SiO₂) isextremely restricted and, therefore, it is difficult in thisglass-ceramic to obtain a coefficient of thermal expansion required inthe present invention and, moreover, since the glass-ceramic is so hardthat it has no good processability. Further, since this glass-ceramicrequires a high temperature of 820° C. to 920° C. for crystallizationwhich prevents a large scale production of the product at a competitivecost.

International Publication WO97/01164 which includes the above describedJapanese Patent Application Laid-open Publication No. Hei 9-35234discloses a glass-ceramic for a magnetic disk in which the lower limitof the Al₂O₃ ingredient is lowered and temperature for crystallizationis reduced (680° C.-770° C.). A sufficient improvement however cannot beachieved by merely lowering the lower limit of the Al₂O₃ ingredient.Besides, crystals grown in all examples disclosed are β-eucriptite(Li₂O.Al₂O₃.2SiO₂) which has a negative coefficient of thermal expansionand, therefore, has the same disadvantage as the above described priorart glass-ceramic.

It is, therefore, an object of the invention to provide a materialsuitable for a substrate for a light filter which has eliminated theabove described disadvantages of the prior art substrate and has athermal expansion property which is sufficient for avoiding variation inthe refractive index at a temperature at which a filter formed with amono-layer or multi-layer film is used (i.e., having a high coefficientof thermal expansion and thereby imparting compressive stress to thefilm to improve temperature stability of the refractive index of thefilm) and also has a mechanical property which imparts sufficientdurability to the filter and further has excellent light transmittance.

SUMMARY OF THE INVENTION

Accumulated studies and experiments made by the inventors of the presentinvention for achieving the above described object of the invention haveresulted in the finding, which has led to the present invention, that,glass-ceramics having, as their predominant crystal phases, lithiumdisilicate (Li₂O.2SiO₂) and α-quartz (α-SiO₂) or α-quartz solid solution(α-SiO₂ solid solution) and having Young's modulus (GPa) of 95 to 120have an excellent processability and is suitable for use as a substratefor a light filter and, more particularly, as a substrate for aband-pass filter or a gain flattening filter.

For achieving the object of the invention, there is provided aglass-ceramic ceramic for a light filter having Young's modulus (GPa)within a range from 95 to 120 and comprising 5.3-8 weight percent ofAl₂O₃, 0.5-3.5 weight percent of ZrO₂ and 71-81 weight percent of SiO₂based respectively on the total content of the oxides.

In one aspect of the invention, the glass-ceramic has specific gravitywithin a range from 2.4 to 2.6.

In another aspect of the invention, the glass-ceramic has a coefficientof thermal expansion which is within a range from 65×10⁻⁷/° C. to130×10⁻⁷/° C. within a temperature range from −50° C. to +70° C.

In another aspect of the invention, predominant crystal phases theglass-ceramic are (a) lithium disilicate (Li₂.2SiO₂) and (b) at leastone of α-quartz (α-SiO₂). and α-quartz solid solution (α-SiO₂ solidsolution).

In another aspect of the invention, the glass-ceramic is substantiallyfree of Na₂O and PbO.

In another aspect of the invention, the glass-ceramic comprises 0.3weight percent or over (expressed on the basis of composition of theoxide) of MgO.

In another aspect of the invention, the glass-ceramic has a compositionwhich comprises in weight percent expressed on the basis of compositionof oxides:

SiO₉ 71-81% Li₂O 8-11% K₂O 0-3% MgO 0.3-2% ZnO 0-1% P₂O₅ 1-3% ZrO₂0.5-3.5% TiO₂ 0-3% Al₂O₃ 5.3-8% Sb₂O₃ 0.1-0.5% SnO₂ 0-5% MoO₃ 0-3% NiO0-2% CoO 0-3%

and has, as predominant crystal phases, (a) lithium disilicate(Li₂O.2SiO₂) and (b) at least one of α-quartz (α-SiO₂). and α-quartzsolid solution (α-SiO₂solid solution).

In another aspect of the invention, the glass-ceramic has, as itspredominant crystal phases, lithium disilicate (Li₂O.2SiO₂) and α-quartz(α-SiO₂) which have fine globular crystal grains.

In another aspect of the invention, average grain diameter of thecrystal phases is 0.30 μm or below.

In another aspect of the invention, the glass-ceramic is obtained bymelting glass materials, forming molten glass, annealing formed glassand then heat treating the formed glass for nucleation under nucleationtemperature within a range from 550° C. to 650° C. for one to twelvehours and further heat treating the formed glass for crystallizationunder crystallization temperature within a range from 680° C. to 800° C.for one to twelve hours.

In another aspect of the invention, there is provided a light filterprovided by forming a multi-layer film on a glass-ceramic as describedabove.

These and other objects and features of the invention will become moreapparent from the description made below.

DETAILED DESCRIPTION OF THE INVENTION

Reasons for limiting the physical properties, surface characteristics,predominant crystal phases and crystal grain diameter, and compositionwill now be described. The composition of the glass-ceramic is expressedin weight percent on the basis of composition of oxides as in their baseglass.

Description will be first made about Young's modulus. As describedabove, as a glass-ceramic used for a light filter which is formed with amulti-layer film thereon, particularly for a band-pass filter or a gainflattening filter, it is preferable for the glass-ceramic to have theYoung's modulus as defined in the claims of the present application fromthe view point of processing and various handling processes. For the useas the light filter, the glass-ceramic is processed to small chips eachhaving a size in the order of, for example, 1 mm×1 mm×1 mm and, ifYoung's modulus is lower than the above defined range, micro-cracking orchipping off of corner portions of these small chips will take place inprocessing of the glass-ceramic to such small chips with resultingsignificant drop in the yield of the product. Micro-cracking or chippingoff of comer portions of small chips does not take place at asignificant rate in processing of the glass-ceramic of the presentinvention presumably by virtue of synergistic effect of a large Young'smodulus and restriction of growth of micro-cracks by precipitatedcrystal grains of the glass-ceramic.

As regards specific gravity, the glass-ceramic should preferably have aslow specific gravity as possible. In most cases where the glass-ceramicis used as a light filter, many small chips, each constituting a lightfilter are mounted on one unit of optical fiber. The light filter madeof the glass-ceramic of the present invention has excellent stability inthe center wavelength of the filter band and also has high wavelengthresolution and, therefore, the unit of optical fiber can receive manywavelengths of light. Accordingly, it is important to reduce the weightof the unit and, for this purpose, specific gravity of the glass-ceramicmust be taken into consideration. If, however, the specific gravity ofthe glass-ceramic is reduced to an excessive degree, it becomesdifficult to achieve a desired Young—s modulus by reason of balance ofratio between precipitated crystal phases and ratio of precipitation ofcrystal phases in the glass-ceramics. Having regard to such balance, ithas been found that the specific gravity should preferably be within arange from 2.4 to 2.6. Having further regard to this balance, it hasbeen found that Young's modulus (GPa)/specific gravity should preferablybe 37 or over and 50 or below.

Coefficient of thermal expansion is a very important factor forimproving the wavelength resolution of the multi-layer film. Morespecifically, stability of center wavelength of a band againsttemperature is very important and, for this purpose, a coefficient ofthermal expansion which is larger than that of a film forming materialis required. As a result of studies and experiments made by theinventors of the present invention, it has been found that, in aband-pass filter, stability of the center wavelength against temperaturedepends to some extent on a refractive index temperature coefficient ofa dielectric which constitutes the thin film and, to a larger extentthan that, on a coefficient of thermal expansion of the substrate. Thisis because refractive index is also determined by a film atomic densityof the thin film. That is, the higher the film atomic density of thethin film is, the smaller becomes variation caused by the temperature ofthe center wavelength. The film atomic density of the thin film isgreatly influenced by the coefficient of thermal expansion of thesubstrate for the light filter on which the thin film is formed. Morespecifically, the temperature of the substrate during the film formingprocess becomes about 200° C. and the substrate thereby is considerablyexpanded. The thin film is formed on this expanded substrate and, as thesubstrate is cooled, the thin film is subjected to compressive stressdue to difference in the coefficient of thermal expansion between them.As a result, the film atomic density of the thin film increases and therefractive index thereby increases. As the coefficient of thermalexpansion of the substrate increases, the compressive stress applied tothe dielectric thin film formed on the substrate increases with theresult that variation in the refractive index due to temperature atwhich the filter is used increases. In a region of the compressivestress above a certain value, variation of refractive index relative tochange in temperature is saturated with a small value of variation. Inother words, by imparting compressive stress above a certain value tothe dielectric thin film, variation in the center wavelength relative tothe temperature becomes constant with a small value of variation. Forthis reason, it is desirable to set the coefficient of thermal expansionof the glass-ceramic at a larger value than the coefficient of thermalexpansion of the dielectric thin film.

It has been found that, if the coefficient of thermal expansion withinthe temperature range from −50° C. to +70° C. is 65×10⁻⁷/° C. or over,sufficient compression stress can be imparted to the film with atemperature range in which the glass-ceramic is used as a band-passfilter and that, if the coefficient of thermal expansion exceeds140×10⁻⁷/° C., differences in the coefficient of thermal expansionbetween the substrate and the filter becomes so large that problems suchas separation of the film from the substrate take place. A preferablerange of the coefficient of thermal expansion is 65×10⁻⁷/° C. to130×10⁻⁷/° C., a more preferable range is 75×10⁻⁷/° C. to 130×10⁻⁷/° C.and the most preferable range is 95×10⁻⁷/° C. to 130×10⁻⁷/° C.

As regards the shape and grain diameter of the percipitated crystalphases, crystal grains and their shape are important factors forachieving the above described characteristics of the glass-ceramic. Thedesired coefficient of thermal expansion cannot be achieved if the graindiameter of the crystal grains of the respective crystal phases is aboveor below the claimed range. The crystal grains should preferably be fineglobular grains from the standpoint of processability and surfaceroughness. More specifically, the crystal grain diameter (average)should preferably be 0.30 μm or below, more preferably less than 0.30 μmand, most preferably, 0.05 μm or over and less than 0.3 μm.

For realizing the above described physical properties and coefficient ofthermal expansion, it has been found that the combination of lithiumdisilicate (Li₂O.SiO₂) and α-quartz (α-SiO₂) as predominant crystalphases is the best combination.

The Na₂O or PbO ingredient is not substantially contained in theglass-ceramic of the invention. Na₂O is an ingredient which posesproblems in forming of the multi-layer film. This is because Na ionsdiffuse in the multi-layer film to deteriorate the properties of thefilm. PbO is an undesirable ingredient from the viewpoint of theenvironment protection. Use of these ingredients, therefore, should beavoided.

Reasons for limiting the composition range of the base glass as definedin the claims will now be described.

The SiO₂ ingredient is a very important ingredient for growing lithiumdisilicate (Li₂O.2SiO₂) and α-quartz (α-SiO₂) as predominant crystalphases by heat treating the base glass. If the amount of this ingredientis below 71%, grown crystals of the glass-ceramic becomes instable andits texture tends to become coarse. If the amount of this ingredientexceeds 81%, difficulty arises in melting and forming of the glass.

The Li₂O ingredient is a very important ingredient for growing lithiumdisilicate (Li₂O.2SiO₂) as a predominant crystal phase by heat treatingthe base glass. If the amount of this ingredient is below 8%, difficultyarises in growing of this crystal phase and also in melting of the baseglass. If the amount of this ingredient exceeds 11%, the grown crystalis instable and its texture tends to become coarse and its chemicaldurability is deteriorated.

The K₂O ingredient improves the melting property of the glass andprevents the grown crystal from becoming too coarse. The amount of up to3% of this ingredient will suffice.

The MgO and ZnO ingredients are ingredients which improve the meltingproperty of the glass and prevent the grown crystals from becomingcoarse and also are effective for enabling the lithium disilicate(Li₂O.2SiO₂), α-quartz (α-SiO₂) and α-quartz solid solution (α-SiO₂solid solution) crystals to grow in the globular form. For this purpose,the amount of the MgO ingredient should preferably be 0.3% or over. Theamount of the ZnO ingredient should more preferably be 0.1% or over. Ifthe amounts of these ingredients are excessive, grown crystals becomeinstable and their textures tend to become coarse. For this reason, theamount of the MgO ingredient should preferably be 2% or less and, morepreferably, 1% or less. Likewise, the amount of the ZnO ingredientshould preferably be 2% or less and, more preferably, be 1% or less. Thesum of the MgO and ZnO ingredients should preferably be 2% or less and,more preferably, 1% or less.

The P₂O₅ ingredient is indispensable as a nucleating agent. If theamount of this ingredient is below 1%, growth of nucleus will becomeinsufficient with resulting abnormal growth of crystals. If the amountof this ingredient exceeds 3%, opaque devitrification will take place inthe base glass.

The ZrO₂ and TiO₂ ingredients are important ingredients which, inaddition to the function, like the P₂O₅ ingredient, as nucleatingagents, are effective for making the grown crystals fine, improving themechanical strength and improving chemical durability. If the amount ofthe ZrO₂ ingredient is below 0.5%, these effects cannot be achieved. Ifthe amount of the ZrO₂ ingredient exceeds 5% or the amount of the TiO₂ingredient exceeds 3%, difficulty arises in melting of the base glassand ZrSiO₄ and the like slug are left unmelted.

The Al₂O₃ ingredient is effective for improving chemical durability andmechanical strength of the glass-ceramic. The type of grown crystaldiffers depending upon conditions of heat treatment. Having regard tovarious conditions of heat treatment, the amount of this ingredientshould be below 10% for growing lithium disilicate (Li₂O.2SiO₂) andα-quartz. A preferable range of this ingredient is 4-8%.

The Sb₂O₃ ingredient is added as a refining agent in melting the baseglass. If the amount of this ingredient is below 0.1%, this effectcannot be achieved. The addition of this ingredient up to 0.5% willsuffice.

The SnO₂ and MoO₃ ingredients may be added because they have anexcellent translucency in the glass state and therefore addition ofthese ingredients facilitate examination of materials beforecrystallization. It will suffice if the amount of the SnO₂ ingredient upto 5% is added and the amount of the MoO₃ ingredient up to 3% is added.

The NiO, CoO, Cr₂O₃ ingredients may be added for adjusting the abovedescribed characteristics of the glass-ceramic within a range notimpairing these characteristics. It will suffice if the amount of theNiO ingredient up to 2%, the amount of the CoO ingredient up to 3% andthe amount of the Cr₂O₃ ingredient up to 3% are added respectively.

Additionally, the glass-ceramic of the invention is required to be freefrom defects such as crystal anisotropy, foreign matters and impuritiesand have a fine and uniform texture and further is required to havemechanical strength and high Young's modulus in processing theglass-ceramic to small chips. The glass-ceramic of the present inventionsatisfies all these requirements.

For manufacturing the glass-ceramic substrate for an information storagemedium according to the invention, glass materials of the abovedescribed composition are melted and is subjected to a hot or coldforming process. The formed glass is subjected to heat treatment under atemperature within a range from 550° C. to 650° C. for one to twelvehours for nucleation and then is subjected to further heat treatmentunder a temperature within a range from 680° C. to 800° C. for one totwelve hours for crystallization.

Predominant crystal phases of the glass-ceramic obtained by the heattreatments are lithium disilicate (Li₂O.2SiO₂) and α-quartz (α-SiO₂)having globular crystal grains with a grain diameter of 0.05 μm or overand 0.30 μm or below.

EXAMPLES

Examples of the present invention will now be described.

Tables 1 to 3 show examples (No. 1 to No. 15) of compositions of theglass-ceramic substrate for an information storage medium made accordingto the invention together with the temperature of nucleation,temperature of crystallization, predominant crystal phases, crystalgrain diameter (average), surface roughness (Ra) after polishing, Rmax,Young's modulus, specific gravity, Young's modulus (GPa)/specificgravity and coefficient of thermal expansion. Table 4 shows compositionsand the above properties of the prior art Si₂—Li₂O—MgO—P₂O₅ systemglass-ceramic disclosed in U.S. Pat. No. 5,626,935 (ComparativeExample 1) and the prior art SiO₂—Al₂O₃—Li₂O system glass-ceramicsdisclosed in Japanese Patent Application Laid-open Publication No.Hei9-35234 (Comparative Example 2) and International Publication No.WO97/01164 (Comparative Example 3).

In the tables, lithium disilicate is abbreviated as “LD” and α-quartz as“α-q”

TABLE 1 Examples 1 2 3 4 5 SiO₂ 75.3 75.5 77.2 77.5 74.3 Li₂O 9.9 9.910.4 9.9 9.5 K₂O 2.0 2.0 2.0 MgO 0.8 1.0 0.5 0.5 0.5 ZnO 0.5 0.5 0.5 0.50.5 P₂O₅ 2.0 2.0 2.0 1.8 2.0 ZrO₂ 2.3 2.3 2.6 2.6 2.0 TiO₂ Al₂O₃ 7.0 6.66.6 7.0 6.0 Sb₂O₃ 0.2 0.2 0.2 0.2 0.2 As₂O₃ SnO₂ 1.5 MoO₃ 1.5 NiO CoOCr₂O₃ Nucleation temperature(° C.) 550 550 550 550 550 Crytallizationtemperature(° C.) 780 770 780 780 780 LD LD LD LD LD Crystal phases and0.10 0.10 0.10 0.10 0.10 grain diameter (average)(μm) α-q α-q α-q α-qα-q 0.20 0.20 0.20 0.20 0.20 Young's modulus(GPa) 100 105 113 120 105Specific gravity 2.47 2.48 2.50 2.52 2.48 Young's modulus(GPa)/specific40 42 45 48 42 gravity Surface roughness (Ra) 7.0 8.0 7.5 6.0 7.3Maximum surface roughness 79.0 83.0 80.4 72.0 81.2 (Rmax) Coefficient ofthermal expansion 110 100 119 123 115 (10⁻⁷/° C.) (−50° C. − +70° C.)

TABLE 2 Examples 6 7 8 9 10 SiO₂ 73.5 71.3 71.3 71.0 73.8 Li₂O 10.0 10.010.0 11.0 9.9 K₂O 1.5 1.5 1.5 1.5 2.0 MgO 0.5 1.0 1.0 1.0 0.8 ZnO 0.50.5 0.5 0.5 0.5 P₂O₅ 2.0 2.0 2.0 2.0 2.0 ZrO₂ 1.5 2.0 2.0 2.0 2.8 TiO₂1.5 1.5 1.5 1.0 Al₂O₃ 6.0 7.0 7.0 6.8 7.0 Sb₂O₃ 0.2 0.2 0.2 0.2 0.2As₂O₃ SnO₂ 1.5 2.0 MoO₃ 1.5 1.0 NiO 0.5 0.5 CoO 1.8 2.0 Cr₂O₃ 0.5 0.5Nucleation temperature(° C.) 560 560 560 590 570 Crytallizationtemperature(° C.) 770 760 780 790 740 LD LD LD LD LD Crystal phases and0.10 0.10 0.10 0.10 0.10 grain diameter (average)(μm) α-q α-q α-q α-qα-q 0.20 0.20 0.05 0.05 0.05 Young's modulus(GPa) 100 115 118 118 100Specific gravity 2.54 2.54 2.53 2.48 2.47 Young's modulus(GPa)/specific39 45 47 48 40 gravity Surface roughness (Ra) 5.5 6.3 5.3 5.0 3.0Maximum surface roughness 63.0 76.0 53.0 51.0 32.0 (Rmax) Coefficient ofthermal expansion 98 100 105 108 95 (10⁻⁷/° C.) (−50° C. − +70° C.)

TABLE 3 Examples 11 12 13 14 15 SiO₂ 75.0 73.1 72.5 78.0 75.1 Li₂O 9.09.5 8.5 8.5 8.5 K₂O 2.5 0.5 0.5 1.0 MgO 1.0 1.5 1.7 1.0 0.5 ZnO 0.8 0.71.0 P₂O₅ 1.5 2.5 1.5 1.5 1.5 ZrO₂ 3.3 0.5 1.0 1.0 1.5 TiO₂ 0.5 2.5 0.51.0 Al₂O₃ 7.4 7.0 8.0 5.3 6.0 Sb₂O₃ 0.1 0.2 0.4 0.5 0.3 As₂O₃ 0.1 0.20.3 0.3 SnO₂ 0.1 4.5 0.3 MoO₃ 2.8 3.0 NiO CoO 2.5 Cr₂O₃ Nucleationtemperature(° C.) 600 570 550 620 560 Crytallization temperature(° C.)750 780 740 780 740 LD LD LD LD LD Crystal phases and 0.10 0.10 0.100.20 0.30 grain diameter (average)(μm) α-q α-q α-q α-q α-q 0.10 0.100.30 0.10 0.05 Young's modulus(GPa) 110 116 118 106 115 Specific gravity2.46 2.56 2.48 2.51 2.49 Young's modulus(GPa)/specific 45 45 48 42 46gravity Surface roughness (Ra) 5.0 4.5 7.0 3.5 3.0 Maximum surfaceroughness 56.0 51.0 88.0 45.0 25.0 (Rmax) Coefficient of thermalexpansion 115 100 130 100 96 (10⁻⁷/° C.) (−50° C. − +70° C.)

TABLE 4 Comparative Examples 1 2 3 SiO₂ 69.0 76.1 76.0 Li₂O 9.0 11.810.0 K₂O 7.0 2.8 2.8 MgO 3.5 ZnO 0.5 P₂O₅ 1.5 2.0 2.0 ZrO₂ 1.0 PbO 1.5Al₂O₃ 5.0 7.1 7.0 BaO 1.5 Sb₂O₃ 0.2 0.2 As₂O₃ 0.5 Nucleationtemperature(° C.) 450 500 450 Crytallization temperature(° C.) 760 850750 LD LD LD Crystal phases and 0.10 0.10 0.10 grain diameter(average)(μm) α-q β-spodumene β-cristobalite 0.60 0.80 0.50 Young'smodulus(GPa) 87 89 90 Specific gravity 2.43 2.53 2.48 Young'smodulus(GPa)/specific 36 35 36 gravity Surface roughness (Ra) 15 17 10Maximum surface roughness 180 230 124 (Rmax) Coefficient of thermalexpansion 64 60 64 (10⁻⁷/°C.) (−50° C. − +70° C.)

For manufacturing the glass-ceramic substrate of the above describedexamples, materials including oxides, carbonates and nitrates are mixedand molten in conventional melting apparatus at a temperature within therange from about 1350 ° C. to about 1450° C. The molten glass is stirredto homogenize it and thereafter formed into a disk shape and annealed toprovide a formed glass. Then, the formed glass is subjected to heattreatment to produce the crystal nucleus under a temperature within therange from 550° C. to 650° C. for about one to twelve hours and then isfurther subjected to heat treatment for crystallization under atemperature within the range from 680° C. to 800° C. for about one totwelve hours to obtain a desired glass-ceramic. Then, this glass-ceramicis lapped with lapping grains having average grain diameter ranging from5 μm to 30 μm for about 10 minutes to 60 minutes and then is finallypolished with cerium oxide having grain diameter ranging from 0.5 μm to2 μm for about 30 minutes to 60 minutes.

The crystal grain shape and grain diameter (average) of the respectivecrystal phases were measured by a transmission electron microscope(TEM). The type of the respective crystal grains were identified by theTEM structure analysis.

The coefficient of thermal expansion was measured according to JOGIS(Japan Optical Glass Industry Standard) 06 and the surface roughness Ra(arithmetic mean roughness) was measured by an atomic force microscope(AFM).

The Young's modulus was measured by the ultrasonic pulse technique ofJIS (Japanese Industrial Standards) R1602 and the specific gravity wasmeasured according to JOGIS 06.

As shown in Tables 1 to 4, the glass-ceramics of the present inventionare different from the comparative examples of the prior artglass-ceramics in the predominant crystal phases and crystal graindiameter (average). In the glass-ceramics of the present invention,crystal grains of lithium disilicate (Li₂O.2SiO₂) and α-quartz (α-SiO₂)are fine globular grains whereas the glass-ceramics of the ComparativeExamples 1, 2 and 3 have a large grain diameter (average) of 0.5 μm orover.

As regards Young's modulus, specific gravity and Young's modulus(GPa)/specific gravity, the glass-ceramics of the present invention haveexcellent Young's modulus (GPa)/specific gravity of 39 or over whereasthe glass-ceramics of Comparative Examples 1, 2 and 3 have Young'smodulus (GPa)/specific gravity of less than 37 and therefore cannotsufficiently cope with a drive of a high speed rotation. Further, asregards the coefficient of thermal expansion, the glass-ceramics of thepresent invention have a coefficient of thermal expansion of 95×10⁻⁷/°C. or over whereas the glass-ceramics of the Comparative Examples 1, 2and 3 have a low coefficient of thermal expansion of 64×10⁻⁷/° C. orbelow. Particularly, the glass-ceramics of Comparative Examples 2 and 3contain β-spodumene and β-cristobalite which are crystal phases having anegative thermal expansion characteristic and, therefore, have a lowthermal expansion characteristic which is insufficient for maintainingstability of the center wavelength of the multi-layer film to thetemperature.

On the glass-ceramic substrates of the above described examples areformed a multi-layer film (e.g., a film of TiO₅/SiO₂, Ta₂O₂/SiO₂ andNb₂O₅/SiO₂) by the sputtering method to provide a light filter. In thelight filter obtained, variation in the center wavelength of transmittedlight relative to temperature is significantly reduced whereby excellentwavelength resolution of the filter can be achieved.

As described in the foregoing, according to the invention, glass-ceramicsubstrates for a light filter having an excellent stability of a centerwavelength to temperature can be provided. The features of theglass-ceramic of the invention, i.e., high light transmittance, highthermal expansion property, high Young's modulus and high bendingstrength, are suitable for an interference type filter, particularly aband-pass filter and are most suitable for WDM and DWDM (densitywavelength division multiplexing) in optical communication systems.Further, the band-pass filter elements which are provided by formingmulti-layer dielectric films of TiO₂/SiO₂, Ta₂O₂/SiO₂ and Nb₂O₅/SiO₂ onthe glass-ceramic substrates of the invention have an excellenttemperature stability of the center wavelength and can be used not onlyfor optical communication systems on the ground but also for space-basedsatellites.

What is claimed is:
 1. A glass-ceramic substrate for a light filterhaving Young's modulus (GPa) within a range from 95 to 120 and surfaceroughness (Ra) of the 8.0 Å or below and comprising 5.3-8 weight percentof Al₂O₃, 0.5-3.5 weight percent of ZrO₂ and 71-81 weight percent ofSiO₂ based respectively on the total content of the oxides.
 2. Aglass-ceramic substrate for a light filter as defined in claim 1 havingspecific gravity within a range from 2.4 to 2.6.
 3. A glass-ceramicsubstrate for a light filter as defined in claim 1 wherein a coefficientof thermal expansion is within a range from 65×10⁻⁷/° C. to 130×10⁻⁷/°C. within a temperature range from −50° C. to +70° C.
 4. A glass-ceramicsubstrate for a light filter as defined in claim 1 wherein a coefficientof thermal expansion is within a range from 65×10⁻⁷/° C. to 140×10⁻⁷/°C. within a temperature range from −50° C. to +70° C.
 5. A glass-ceramicsubstrate for a light filter as defined in claim 1 wherein predominantcrystal phases are (a) lithium disilicate (Li₂O.2SiO₂) and (b) at leastone of α-quartz (α-SiO₂). and α-quartz solid solution (α-SiO₂ solution).6. A glass-ceramic substrate for a light filter as defined in claim 1which is substantially free of Na₂O and PbO.
 7. A glass-ceramicsubstrate for a light filter as defined in claim 1 which comprises 0.3weight percent or over of MgO based on the total content of the oxides.8. A glass-ceramic substrate for a light filter as defined in claim 1having a composition which comprises in weight percent based on thetotal content of the oxides: SiO₂ 71-81% Li₂O 8-11% K₂O 0-3% MgO 0.3-2%ZnO 0-1% P₂O₅ 1-3% ZrO₂ 0.5-3.5% TiO₂ 0-3% Al₂O₃ 5.3-8% Sb₂O₃ 0.1-0.5%SnO₂ 0-5% MoO₃ 0-3% NiO 0-2% CoO 0-3% Cr₂O₃ 0-3%

and having, as predominant crystal phases, (a) lithium disilicate(Li₂O.2SiO₂) and (b) at least one of α-quartz (α-SiO₂). and α-quartzsolid solution (α-SiO₂ solid solution).
 9. A glass-ceramic substrate fora light filter as defined in claim 1 having, as its predominant crystalphases, lithium disilicate (Li₂O.2SiO₂), α-quartz (α-SiO₂) and α-quartzsolid solution (α-SiO₂ solid solution) which have fine globular crystalgrains.
 10. A glass-ceramic substrate for a light filter as defined inclaim 1 wherein average grain diameter of the crystal phases is 0.30 μmor below.
 11. A glass-ceramic substrate for a light filter as defined inclaim 1 obtained by melting glass materials, forming molten glass,annealing formed glass and then heat treating the formed glass fornucleation under nucleation temperature within a range from 550° C. to650° C. for one to twelve hours and further heat treating the formedglass for crystallization under crystallization temperature within arange from 680° C. to 800° C. for one to twelve hours.
 12. A lightfilter provided by forming a multi-layer film on a glass-ceramicsubstrate for a light filter as defined in claim
 1. 13. A glass-ceramicsubstrate for a light filter having a Young's modulus (Gpa) within arange from 95 to 120, containing, as a predominant crystal phase,α-quartz (α-SiO₂) having an average grain diameter of 0.2 μm or below,and comprising 5.3-8 weight percent of Al₂O₃, 0.5-3.5 weight percent ofZrO₂ and 71-81 weight percent of SiO₂ based respectively on the totalcontent of the oxides.
 14. A glass-ceramic substrate for a light filteras defined in claim 13 having specific gravity within a range from 2.4to 2.6.
 15. A glass-ceramic substrate for a light filter as defined inclaim 13 wherein a coefficient of thermal expansion is within a rangefrom 65×10⁻⁷/° C. to 130×10⁻⁷/° C. within a temperature range from −50°C. to +70° C.
 16. A glass-ceramic substrate for a light filter asdefined in claim 13 wherein a coefficient of thermal expansion is withina range from 65×10⁻⁷/° C. to 140×10⁻⁷/° C. within a temperature rangefrom −50° C. to +70° C.
 17. A glass-ceramic substrate for a light filteras defined in claim 13 wherein predominant crystal phases are (a)lithium disilicate (Li₂O.2SiO₂) and (b) at least one of α-quartz(α-SiO₂) and α-quartz solid solution (α-SiO₂ solution).
 18. Aglass-ceramic substrate for a light filter as defined in claim 13 whichis substantially free of Na₂O and PbO.
 19. A glass-ceramic substrate fora light filter as defined in claim 13 which comprises 0.3 weight percentor over of MgO based on the total content of the oxides.
 20. Aglass-ceramic substrate for a light filter as defined in claim 13 havinga composition which comprises in weight percent based on the totalcontent of the oxides: SiO₂ 71-81% Li₂O  8-11% K₂O 0-3% MgO 0.3-2%   ZnO0-1% P₂O₅ 1-3% ZrO₂ 0.5-3.5% TiO₂ 0-3% Al₂O₃ 5.3-8%   Sb₂O₃ 0.1-0.5%SnO₂ 0-5% MoO₃ 0-3% NiO 0-2% CoO 0-3% Cr₂O₃ 0-3%

and having, as predominant crystal phases, (a) lithium disilicate(Li₂O.2SiO₂) and (b) at least one of α-quartz (α-SiO₂) and α-quartzsolid solution (α-SiO₂ solid solution).
 21. A glass-ceramic substratefor a light filter as defined in claim 13 having, as its predominantcrystal phases, lithium disilicate (Li₂O.2SiO₂), α-quartz (α-SiO₂) andα-quartz solid solution (α-SiO₂ solid solution) which have fine globularcrystal grains.
 22. A glass-ceramic substrate for a light filter asdefined in claim 13 obtained by melting glass materials, forming moltenglass, annealing formed glass and then heat treating the formed glassfor nucleation under a nucleation temperature within a range from 550°C. to 650° C. for one to twelve hours and further heat treating theformed glass for crystallization under crystallization temperaturewithin a range from 680° C. to 800° C. for one to twelve hours.
 23. Alight filter provided by forming a multi-layer film on a glass-ceramicsubstrate for a light filter as defined in claim 13.